The present invention generally relates to optical movement measuring methods and apparatuses, and more particularly to an optical movement measuring method which is applicable to various encoders and the like for measuring micromovements or microdisplacements and an optical movement measuring apparatus which employs this optical movement measuring method.
It is known to irradiate a light which is emitted from a light source onto an object and detect a position of the object by detecting a reflected light or a transmitted light from the object using a photodetector.
A method of optically measuring a movement of an object was proposed in a Japanese Laid-Open Patent Application No. 63-47616. According to this proposed method, an object having a periodic structure is illuminated by a divergent bundle of rays emitted from a monochromatic point light source so as to generate a shadow diffraction pattern which corresponds to the periodic structure of the object. A photodetector detects a movement of the object by detecting a movement of the shadow diffraction pattern which moves as the object moves in a direction traversing the divergent bundle of rays.
However, this proposed method suffers the following problems. That is, as is clear from a logical analysis disclosed in the Japanese Laid-Open Patent Application No. 63-47616, it is necessary to realize a predetermined positional relationship among the point light source, the object and the photodetector in order to generate the shadow diffraction pattern. When this positional relationship changes, the generated shadow diffraction pattern also changes. This means that the shadow diffraction pattern changes and an error is introduced in the measured movement when the positional relationship changes for some reason or simply changes with time.